Adaptations

Water lilies, a kind of adaptations

The results of natural selection in which succeeding generations of organisms become better able to live in their environments are called adaptations. Many of the features that are most interesting and beautiful in biology are adaptations. Specialized structures, physiological processes, and behaviors are all adaptations when they allow organisms to cope successfully with the special features of their environments.

Adaptations ensure that individuals in populations will reproduce and leave well-adapted offspring, thus ensuring the survival of the species. Adaptations arise through mutations—inheritable changes in an organism’s genetic material.

These rare events are usually harmful, but occasionally they give specific survival advantages to the mutated organism and its offspring. When certain individuals in a population possess advantageous mutations, they are better able to cope with their specific environmental conditions and, as a result, will contribute more offspring to future generations than those individuals that lack the mutation.

Over time, the number of individuals that have the advantageous mutation will increase in the population at the expense of those that do not have it. Individuals with an advantageous mutation are said to have a higher fitness than those without it, because they tend to have comparatively higher survival and reproductive rates. This is natural selection.

Natural Selection

Over very long periods of time, evolution by natural selection results in increasingly better adaptations to environmental circumstances. Natural selection is the primary mechanism of evolutionary change, and it is the force that either favors or selects against mutations.

Growing on poisonous environtment

Although natural selection acts on individuals, a population gradually changes as those with adaptations become better represented in the total population. Most flowering plants, for example, are unable to grow in soil containing high concentrations of certain elements (for example, heavy metals) commonly found in mine tailings.

Therefore, an adaptation that conferred resistance to these elements would open up a whole new habitat where competition with other plants would be minimal. Natural selection would favor the mutations, which confer specific survival advantages to those that carry them and impose limitations on individuals lacking these advantages.

Thus, plants with special adaptations for resistance to the poisonous effects of heavy metals would have a competitive advantage over those that find heavy metals toxic. These attributes would be passed to their more numerous offspring and, in evolutionary time, resistance to heavy metals would increase in the population.

Types of Adaptations

Although natural selection serves as the instrument of change in shaping organisms to very specific environmental features, highly specific adaptations may ultimately be a disadvantage. Adaptations that are specialized may not allow sufficient flexibility (generalization) for survival in changing environmental conditions.

The degree of adaptative specialization is ultimately controlled by the nature of the environment. Environments, such as the tropics, that have predictable, uniform climates and have had long, uninterrupted periods of climatic stability are biologically complex and have high species diversity.

Scientists generally believe that this diversity results, in part, from complex competition for resources and from intense predator-prey relationships. Because of these factors, many narrowly specialized adaptations have evolved when environmental stability and predictability prevail.

By contrast, harsh physical environments with unpredictable or erratic climates seem to favor organisms with general adaptations, or adaptations that allow flexibility. Regardless of the environment type, organisms with both general and specific adaptations exist because both types of adaptation enhance survival under different environmental circumstances.

Metabolism is the sum of all chemical reactions taking place in an organism, whereas physiology consists of the processes involved in an organism carrying out its function. Physiological adaptations are changes in the metabolism or physiology of organisms, giving them specific advantages for a given set of environmental circumstances.

Because organisms must cope with the rigors of their physical environments, physiological adaptations for temperature regulation, water conservation, varying metabolic rate, and dormancy allow organisms to adjust to the physical environment or respond to changing environmental conditions.

Adaptations and Environment

Desert environments, for example, pose a special set of problems for organisms. Hot, dry environments require physiological mechanisms that enable organisms to conserve water and resist prolonged periods of high temperature.

Evolution has favored a specialized form of photosynthesis in cacti and other succulents inhabiting arid regions. Crassulacean acid metabolism (CAM) photosynthesis allows plants with this physiological adaptation to absorb carbon dioxide at night, when relative humidity is comparatively high and air temperatures relatively low.

Adaptations and Environment

Taking in carbon dioxide during the day would dehydrate plants, because opening the pores through which gas exchange takes place allows water to escape from the plant. CAM photosynthesis, therefore, allows these plants to exchange the atmospheric gases essential for their metabolism at night, when the danger of dehydration is minimized.

Because organisms must also respond and adapt to an environment filled with other organisms— including potential predators and competitors— adaptations that minimize the negative effects of biological interactions are favored by natural selection. Often the interaction among species is so close that each species strongly influences the others and serves as the selective force causing change.

Under these circumstances, species evolve together in a process called coevolution. The adaptations resulting from coevolution have a common survival value to all the species involved in the interaction. The coevolution of flowers and their pollinators is a classic example of these tight associations and their resulting adaptations.

Speciation

Adaptations can be general or highly specific. General adaptations define broad groups of organisms whose lifestyles are similar. At the species level, however, adaptations are more specific and give narrow definition to those organisms that are more closely related to one another.

Slight variations in a single characteristic, such as bill size in the seed-eating Galapágos finches, are adaptive in that they enhance the survival of several closely related species. An understanding of how adaptations function to make species distinct also furthers the knowledge of how species are related to one another.

Why so many species exist is one of the most intriguing questions of biology. The study of adaptations offers biologists an explanation. Because there are many ways to cope with the environment, and because natural selection has guided the course of evolutionary change for billions of years, the vast variety of species existing on the earth today is simply an extremely complicated variation on the theme of survival.